This invention relates to oscillator circuits and, more specifically, to oscillator circuits of the type which contain a diode in a timing circuit that controls the oscillator frequency.
One type of relaxation oscillator adapted to use this invention generally comprises a unijunction transistor, a timing circuit and a voltage source. The unijunction transistor has an emitter electrode and base 1 and 2 electrodes. Generally, the voltage source connects across the base electrodes. The timing circuit comprises a resistor and capacitor in series. The resistor connects between the power supply and the emitter electrode while the capacitor connects in parallel with the emitter and base 1 electrodes. The unijunction transistor may be any type of unijunction transistor. When the voltage of the capacitor exceeds a peak voltage, the unijunction transistor discharges the capacitor through the emitter-base junction which is a diode.
Another oscillator circuit of this type is a free running multivibrator. The free running oscillator comprises first and second switching transistors which conduct alternately thereby to control the charging and discharging of timing capacitors in a timing circuit that includes the base-emitter junctions of the transistors. Again, these junctions constitute diode junctions in the timing circuit.
Oscillators of this type are not stable with respect to temperature variations, and the frequency of the oscillator can vary significantly with temperature variations. This thermal instability is due almost entirely to the temperature sensitivity of the voltage across these diodes in the timing circuits.
In the prior art, compensation for the unijunction transistor oscillator is generally provided by inserting a temperature compensating resistor in series with the base 2 electrode. The value of this compensating resistor is based on the intrinsic standoff ratio of the unijunction transistor and the value of the interbase resistance. This use of a compensating resistor is based on the fact that the inter-base resistance (R.sub.BB) between the base terminals of the unijunction transistor increases with temperature at approximately 0.8% per .degree. C. The compensating resistor is selected so that as R.sub.BB varies, the base-to-base voltage, V.sub.BB, also varies. The term .eta. V.sub.BB then varies and approximately compensates for changes in voltage across the diode junction.
With this compensation, frequency variations can be maintained to .+-.1% over a range from -20.degree. C to +70.degree. C. If this variation in frequency is not acceptable, the prior art suggests that the compensating resistor be adjusted for a minimum variation under cyclical heating and cooling by using a temperature chamber. These operations are extremely time consuming and add expense to the oscillator circuit.
Furthermore, the compensation results are valid only for one given frequency. If the frequency changes, the required value for the compensating resistor also changes. Thus, these prior techniques do not compensate a variable-frequency oscillator incorporating unijunction transistors.
Various temperature compensation schemes for free-running multivibrators also appear in the prior art. In all of these a differential voltage source is inserted in the timing circuit while the power supply voltage is held constant. Typical differential voltage sources include a transistor coupled to the base-emitter junction of one multivibrator transistor, a voltage generator comprising a diode circuit coupled to the collectors of the multivibrator transistors and circuitry for supplying a compensating voltage to the base electrodes of the multivibrator transistors.
Thus, all the temperature compensation circuits for relaxation and free running multivibrator oscillators have common characteristics. First, the supply voltage is held constant and isolated from the active element (i.e., the unijunction transistor or the multivibrator transistors) in the timing circuit. Secondly, a source of an approximate compensating voltage is superimposed on normal timing circuit signals. Although these temperature compensation techniques tend to reduce frequency variations due to temperature changes, the variations still remain to a significant degree. In one of the multivibrator examples, a variation of over 100 parts per million per degree Celsius still exists.
Therefore, it is an object of this invention to provide a solid-state oscillator which produces a stable output frequency over a wide range of temperatures and input voltages.
Another object of this invention is to provide a temperature compensation circuit for solid-state oscillators which is relatively inexpensive.
Still another object of this invention is to provide a unijunction transistor relaxation oscillator which produces a stable output frequency over a wide range of temperatures and input voltages.
Yet another object of this invention is to provide unijunction relaxation oscillator which produces a stable output frequency over a wide range of frequencies.
Yet still another object of this invention is to provide a free-running multivibrator oscillator which produces a stable output frequency over a wide range of temperatures and input voltages.